The present disclosure relates to smart windows, also called dynamic glazing, and the use of electrofluidic devices to change the transmission of the solar spectrum through a window, transparent surface and/or translucent surface.
Electrowetting has been a highly attractive modulation scheme for a variety of optical applications. For example, electrowetting has been used to provide optical switches for fiber optics, optical shutters or filters for cameras and guidance systems, optical pickup devices, optical waveguide materials and video display pixels.
Conventional electrowetting displays include colored oil that forms a film layer against an electrically insulating fluoropolymer surface. Underneath the fluoropolymer is a reflective electrode constructed from aluminum. This colored oil film layer provides coloration to the reflective surface below. When a voltage is applied between a water layer residing above the oil film layer and the electrode below the fluoropolymer, the oil film layer is broken up as the water electrowets the fluoropolymer. When the voltage is removed, the oil returns to the film layer geometry. While the oil film layer is broken up, the perceived coloration of the surface is that of the reflective electrode (e.g., white) whereas, when the oil is in the film state, the perceived coloration is that of the oil. Coloration of the oil is provided by including at least one dye. Conventional electrowetting technology may provide greater than 70% white state and a contrast ratio of up to 10:1 in the visible spectrum for the purpose of information displays.
Incident sunlight brings about 1 KW/m2 of energy to the earth's surface over the wavelength range of 300 to 2500 nm, with ˜52% of that energy being in infrared wavelengths. A majority of infrared energy lies in the Infrared-A range (700-1400 nm), with nearly 50% of incident infrared energy and 25% of total solar energy lying in the 700-1000 nm range. Buildings are generally designed to maintain a constant and comfortable temperature and employ heating and cooling systems to do so. Energy efficient buildings often make use of solar radiation for both lighting needs and temperature control.
Buildings today use passive techniques to achieve the energy efficiency status quo. Windows provide lighting, and insulated walls, roofs, windows and skylights are designed to isolate the indoor climate from the outdoor climate. Passive paints have been developed to reflect infrared from buildings. These contain infrared-reflecting pigments, or infrared-transparent pigments combined with visible-region pigments on an infrared-reflecting substrate. In addition, some buildings employ designs that restrict the high summer sun from entering southern exposure windows, but allow lower winter sunlight to come through them, providing some seasonal adaptability. While some structures take advantage of these designs, new track home developments, for example, place the same several floor plans on each lot regardless of sun orientation. Consequently, while efficient passive components are available, efficient design and implementation is not necessarily reaching the bulk of the population. Federal Energy Star guidelines have attempted to set performance levels in order to drive improved efficiency.
Several technology-based solutions for manipulating solar heat gain and minimizing thermal heat transfer have been successfully deployed in recent decades. For example, low emissivity coatings applied to the inside surface(s) of dual pane windows restrict thermal transfer across the insulating gas gap. In cool climates, windows have this film on the inside pane to keep in heat. In hot climates, this coating lies of the outside glass sheet to keep the heat on the outside. In addition to these coatings, window glass and window films are utilized to reflect or absorb solar heat gain prior to that energy entering the building, but only for those climates where excessive heat is the dominant issue. In addition to these passive window coatings, infrared-reflective pigments have been developed for applications such as greenhouses and roof tiles. These pigments have been tailored to reflect infrared while impacting the desirable aspects visible spectrum as little as possible. They are sometimes used on roofs in sunny climates and siding on southern exposures.
A key weakness of passive techniques for controlling solar heat gain is that a one-sized solution does not fit all. In many climates, solar heat gain should be maximized in the winter, but minimized in the summer. In dry climates where the temperature varies by 30 degrees each day, the solar heat gain should be maximized in the 45° F. morning and minimized in the 75° F. afternoon.
Federal Energy Star guidelines set performance requirements for windows in different climate zones within the United States that can generally only be achieved with dual pane window designs and passive windows coatings that reduce transmission of visible and infrared light. Energy Star divides the United States into four regions. Windows built for Napa Valley have the same coating requirements as those built for El Paso and Atlanta, even though the climates are remarkably different. Wherever we can better match the diurnal, seasonal, and regional solar heat gain to a building's needs, we improve our energy efficiency.
Active solutions promise further improvements. Clearly, some active solutions have been in place for time eternal, like closing curtains and blinds on hot days to keep out heat and on winter nights to keep in heat. For example, Phoenix residents may put up attenuating sun screens in the summer, while Boston residents may tape plastic over windows in the winter. Some have employed sun-tracking reflective solar curtains. Others have installed existing smart window technologies, existing smart window technology. However some of this technology has disadvantages that have prevented widespread adoption.
There are generally three main types of smart window technology, polymer-dispersed liquid crystal, electrochromic, and suspended particle. Polymer-dispersed liquid crystal is generally comprised of spheres of encapsulated liquid crystal. Without power, the liquid crystal molecules are randomly organized and scatter light, created a translucent (or privacy) surface. With voltage, the liquid crystal molecules align themselves to the direction of window transmission, allowing light to pass through without scattering. Polymer-dispersed liquid crystal is not hardy enough for sun-facing applications due to the UV-instability of liquid crystal material, and is used primarily for interior privacy applications. Suspended particle windows contain elongated light absorbing micro-crystals between two transparent sheets. With no power, these crystals are randomly-aligned and absorb all the incident light. With a voltage applied between the two transparent sheets, the elongated crystals line up perpendicular to the sheet surface, allowing about 50% of light to pass through. Suspended particle windows are dark with no applied power, and hence become dark during power failure. This restricts their application making them unsuitable for windshields in autos, boats, planes and buses, and for some architectural applications. These types of windows are also heavily tinted in the ‘clear’ state. Electrochromic windows contain a chemical gel and a metal oxide that is transparent. Application of voltage causes a chemical reaction in the gel that removes oxygen from the oxide causing it to absorb light. Electrochromic windows switch from clear to dark very slowly (minutes) and are difficult to implement on curved surfaces. While these technologies may have uses, neither suspended particle nor electrochromic technologies shade visible light while transmitting infrared, which is generally desirable for the northern climate zone. Further, neither electrochromic nor suspended particle can modulate the reflection of infrared wavelengths, which is desirable for southern climates.
Other optical shutter technologies used in displays are incompatible with infrared modulation as well. For example, electrophoretic modules often incorporate titania as an optically white material. Titania is also an infrared reflector, so electrophoretic modules reflect infrared at the same time that they reflect visible light. However, electrophoretic modules such as E-Ink are not transparent and cannot be used as windows. Finally, cholesteric liquid crystal modules have been demonstrated which reflect some infrared light over a fairly narrow range, and contain multiple layers to independently reflect various colors. However, the extreme UV sensitivity of these materials, combined with the narrow reflectivity ranges of each layer, make these intractable for exterior-facing window applications.
Another optical shutter technology that is incompatible with selectable infrared reflection or selectable infrared transmission with visible attenuation is electrowetting. Conventional electrowetting moves a dyed oil across a surface in the presence of a polar fluid, in order to modulate transmission. In the case of infrared reflection, dyes absorb, rather than reflect, so reflection by the fluid is not possible. Moreover, the particles that are needed to scatter or reflect light are generally incompatible with the oil (or non-polar) phase because dispersing these particles leads to electrophoretic or conductive properties of the fluid that is supposed to be non-polar and insulating. These particles and dispersants spoil the electrowetting property. In the case of infrared transmission, but visible absorption, dyes exist which are black but infrared transmissive. However, dyes degrade in the Sun orders of magnitude faster than pigments, and are simply not suitable for the application. Thus, conventional electrowetting technology using dyes cannot meet the requirements for windows.
What is lacking is an inexpensive technology to actively manage the total solar spectrum (e.g., near infrared and visible) through windows by day, season and region. Moreover, it may be desirable to independently manage the infrared and visible regions, because consumers often want visible light without the infrared heat or infrared heat without the visible light.